Well that brings back memories, many involving a tow rope!. We bought an Alpine twin track that had a 640 Rotax (in pieces) with a broken crank for a project some time back that also came with a 641 Rotax crankcase assy., so with some work it was all fixed up and running great(had no problem spinning both tracks taking off). However It could not make it up and down the mountain without blowing a hole in the no. 2 piston, so after some standard pistons had to hone it oversize and then after more oversize pistons (all that where in Canada at the time!) I was getting very tired of rebuilding it!. Finally We removed the rebuilt original carb., got an adaptor and fitted a modern Mikuni carb+pump and No more problems!. The old carb. would randomly lean-out for no reason and was not noticeable till too late when wide open which with the low gearing is where you spent most of the time!, so after some good runs it was sold to erase the memories . Consider your self warned. Nathan.